1. Field of the Invention
The present invention relates to signal processing for enhancing the retrieval of data from magnetic tape.
2. Background Art
Data is stored on a magnetic tape as variations in the magnetic field produced by particles in the tape. This data is typically read by sensing these fields with a tape head. As the tape travels past the tape head, variations in the magnetic field induce a time-varying voltage in the head. The analog signals generated represent the data stored on the tape. The typical tape reader also incorporates pre-amplifiers, analog processors, analog-to-digital converters, and the like to process tape head output signals.
Increases in tape data storage capacity necessitate an increase in signal data density. However, increases in signal data density may decrease performance due to reduced signal-to-noise ratios. In addition, partial response channel systems depend heavily on inter-symbol interference to function properly. Such systems achieve this interference by passing the received signal through a pulse shaping filter matching the signal spectral response to channel requirements. However, variations in the head-to-tape positioning alter the signal spectral components. The resulting mis-equalization causes increasing signal degradation.
Low density parity check decoding is an evolving technology for retrieving data from magnetic storage media through partial response channels. Low density parity check decoding uses an iterative approach based on multiple passes of a signal containing data through a decoder. The information learned in each decoder pass is used in subsequent decoder passes. Iterations continue until the data is fully decoded from the signal or the design maximum number of decoder passes is reached. Low density parity check decoding can theoretically achieve an effective increase in signal strength of 5 dB, thus compensating for reduced signal amplitudes associated with higher signal data density recording.